The present invention generally relates to fabrication processes that include a joining operation. More particularly, this invention is directed to a technique for fabricating rotating hardware, as an example, rotating components of a turbomachine, joining techniques used in their fabrication, and the hardware formed thereby.
Components within the combustor and turbine sections of a gas turbine engine are often formed of superalloy materials in order to achieve acceptable mechanical properties while at elevated temperatures resulting from the hot combustion gases produced in the combustor. Higher compressor exit temperatures in modern high pressure ratio gas turbine engines can also necessitate the use of high performance superalloys for compressor components, including blades, spools, disks (wheels) and other components. Suitable alloy compositions and microstructures for a given component are dependent on the particular temperatures, stresses, and other conditions to which the component is subjected. For example, the rotating hardware such as compressor spools, compressor disks, and turbine disks are typically formed of superalloys that must undergo carefully controlled forging, heat treatments, and surface treatments to produce a controlled grain structure and desirable mechanical properties. Notable superalloys for these applications include gamma prime (γ′) precipitation-strengthened nickel-base superalloys containing chromium, tungsten, molybdenum, rhenium and/or cobalt as principal elements that combine with nickel to form the gamma (γ) matrix, and contain aluminum, titanium, tantalum, niobium, and/or vanadium as principal elements that combine with nickel to form the desirable gamma prime precipitate strengthening phase, principally Ni3(Al,Ti). Examples of gamma prime nickel-base superalloys include René 88DT (R88DT; U.S. Pat. No. 4,957,567) and René 104 (R104; U.S. Pat. No. 6,521,175), as well as certain nickel-base superalloys commercially available under the trademarks Inconel®, Nimonic®, and Udimet®. Disks and other critical gas turbine engine components are often forged from billets produced by powder metallurgy (P/M), conventional cast and wrought processing, and spraycast or nucleated casting forming techniques. Forging is typically performed on fine-grained billets to promote formability, after which a supersolvus heat treatment is often performed to cause uniform grain growth (coarsening) to optimize properties.
A turbine disk 10 of a type known in the art is represented in FIG. 1. The disk 10 generally includes an outer rim 12, a central hub 14, and a web 16 between the rim and hub 12 and 14. The rim 12 is configured for the attachment of turbine blades (not shown) in accordance with known practice. A hub bore 18 in the form of a through-hole is centrally located in the hub 14 for mounting the disk 10 on a shaft, and therefore the axis of the hub bore 18 coincides with the axis of rotation of the disk 10. The disk 10 is presented as a unitary forging of a single alloy, and is representative of turbine disks used in aircraft engines, including but not limited to high-bypass gas turbine engines such as the GE90® and GEnx® commercial engines manufactured by the General Electric Company. The weight and cost of single-alloy forgings have driven the desire to develop materials, fabrication processes, and hardware designs capable of reducing forging weight and costs for rotating hardware of gas turbines. One approach is prompted by the fact that the hubs and webs of compressor spools and disks and turbine disks have lower operating temperatures than their rims, and therefore can be formed of alloys with properties different from those required at the rims. Depending on the particular alloy or alloys used, optimal microstructures for the hub, web and rim can also differ. For example, a relatively fine grain size may be optimal for the hub and web to improve tensile strength and resistance to low cycle fatigue, while a coarser grain size may be optimal in the rim for improving creep, stress-rupture, and crack growth resistance.
Implementing a multi-alloy design generally entails separately fabricating the hub and rim of a disk from different materials and then joining the hub and rim by welding or another metallurgical joining process, as disclosed in U.S. Published Patent Application Nos. 2008/0120842 and 2008/0124210. Though a variety of joining techniques are available for producing multi-alloy disks, each has certain shortcomings. For example, electron beam (EB) welding creates a resolidified weld zone that is always weaker than the materials welded together, and joints formed by diffusion bonding (DB) and brazing are also weaker than the materials they join as a result of providing no mechanical work to the joint region. Solid-state welding processes such as inertia welding are disclosed in U.S. Pat. No. 6,969,238. While well suited for certain applications, weld joints formed by inertia welding are fine grained and therefore limit the high temperature operation of a disk. Furthermore, if the disk is heat treated to produce coarser grain size, the inertia weld joint is prone to cracking and critical grain growth during supersolvus heat treatment.
Further examples of metallurgical joining techniques for fabricating multi-alloy disks and spools are disclosed in U.S. Pat. Nos. 5,106,012 and 5,161,950. These patents describe a technique termed forge enhanced bonding, by which separately formed regions of a disk can be bonded together during a forging operation. In a particular example, preforms of the rim region and the hub and web region of a disk are placed in a forging die and bonded together during forging as a result of material at the interface of the preforms being displaced into vents in the die halves. Potential defects originally present at the interface surfaces are displaced with the material that flows into the vents, forming sacrificial ribs that can be removed from the resulting bonded disk after forging, so that the portion of the bond line remaining in the finish part is of high integrity and substantially free from defects. While effective for bonding hub and rim preforms, the process requires producing the preforms so that their mating surfaces are very clean and closely shape-conforming, carefully assembling the preforms in a can while avoiding contamination, and hot isostatic pressing (HIP) the preforms prior to forging.